A very risky move, destabilizing the pawns on the kingside.
12... a7a5 It's necessary to stop a4-a5. 13. b3c5 White has a few tempting moves: 13... b6d5  14. c5e4 The knight is ready to go to d6. There are three ways to cover this square, and all are reasonable. 14... g8f8 